For simple note-taking, the built-in Notes app in OS X is adequate enough for most (if you're looking for something more full-featured, check out Evernote). The Notes app is also great for syncing between all your devices because it uses iCloud for transparent/background syncing. I'm guessing that you already know this.
What you may not know, however, is that you can also store images within your notes, and these images (as you would imagine) also sync between all your devices. This is how it works:
Simple? Absolutely. Powerful? You bettcha!
For the record, I actually use the Notes app quite heavily. I find it to be simple and very reliable. The one downside from my perspective... no roll-back functionality (in the event that I accidentally delete a note).